This invention relates generally to radar processing systems, and more particularly to pulse compression systems yielding low range-time sidelobes.
It is well known in the art of pulse radar systems that in order to obtain a good detection capability against a background of noise, a pulse with a large energy content must be transmitted. This large energy content may be obtained by either transmitting a pulse with a large peak power and/or with a long pulse duration. If the pulse width is limited to small values because of the desire to obtain good range accuracy or resolution, the required energy pulse must be obtained with a large peak power. However, in many applications it is not possible to obtain a peak power as large as one might desire because of voltage-peak limitations somewhere in the system. In such peak-power-limited radar systems, the required energy can be obtained only by transmitting a longer pulse. In order to retain radar range resolution when transmitting a long pulse with a high average power content, pulse compression techniques are employed. The use of such pulse compression techniques permit the transmitted pulse to be made as long as desired while retaining an optimum range resolution. In the pulse compression process, a long coded pulse with a band-width B much greater than the reciprocal of the pulse length is transmitted and echo returns therefrom are decoded to form short pulses with durations equal to 1/B. The codes employed can be either phase codes or frequency codes. The phase codes are generated by transmitting a phase for a time interval 1/B, changing the phase for the next time interval 1/B, etc. It has been found that the use of the Frank phase code (R. L. Frank, "Polyphase Codes With Good Nonperiodic Correlation Properties," IEEE Trans. IT-9,43-45, Jan. 1963) is especially advantageous because it has excellent doppler tolerance and very low range-time sidelobes (the highest sidelobe is approximately p.pi..sup.2 down from the mainlobe, where p is the pulse compression ratio). These low sidelobes are achieved without amplitude weighting, as required in many other types of codes. The low range-time sidelobes are especially important in order to minimize the radar output when a echo return is not properly indexed in the radar compressor. The decoding of a Frank coded transmission may be accomplished by means of a Fast Fourier Transform circuit thereby reducing the number of processing elements from N.sup.2 to N Log.sub.2 N. Thus, the ability to use Fast Fourier Transform processing yields a significant cost reduction in the system. See U.S. Pat. No. 4,237,461 B. H. Cantrell and B. L. Lewis for a "High-Speed Digital Pulse Compressor." However, the abrupt 0 to .pi. phase shifts normally used to implement the sharp steps in the Frank code require a transmitter, receiver, and processing circuitry with a very broad band frequency characteristic. Such broad band systems with bandwidths significantly larger than the reciprocal of the compressed pulse duration allow in a significant amount of out-of-band thermal noise which will fold-over when the signal is sampled at the Nyquist rate for conversion to I and Q digital words. This noise fold-over seriously degrades the signal-to noise ratio of the radar thereby reducing target detectability. Such broad bandwidth systems are also expensive. Thus, it is generally desired to match the receiver bandwidth and the subsequent I.F. amplifying circuits to a bandwidth on the order of the reciprocal of the compressed pulse duration. However, if the receiver and/or processing circuitry bandwidth is not wide, then the abrupt phase changes in the phase modulation result in amplitude modulation which causes the properly indexed output and the sidelobes to vary drastically with target range in cycles of length C/2B (the sampling rate), where C is the velocity of light. This amplitude modulation is due to the fact that the period of time required for a circuit such as an I.F. amplifier to build up to a given amplitude is inversely proportional to the bandwidth of the circuit. A 0 to .pi. phase shift requires the signal amplitude of the amplifier to go to zero and then to build up to the same signal amplitude but with a .pi. phase. Thus, when abrupt phase changes of .pi. radians occur in a code, as they do in the midsection of the Frank code, the band limited receiver and/or I.F. processing circuitry must de-ring and re-ring with the amplitude of the response going through zero with a non-infinite slope. If the system samples during this 0 to .pi. phase change, the sampled signal could have any value from zero to the proper signal amplitude.
The spectrum of a decoded pulse in Frank code generally takes the form of a (sin x)/x curve in a wideband system. The use of a restricted bandwidth receiving system limits this usable bandwidth to a portion 3 dB down on either side of the mainlobe as shown in FIG. 1. In essence, because of the finite time required for the narrow bandwidth system to respond to abrupt signal changes, the signal amplitude tends to be averaged or spread across a series of coutiguous range cells. Thus, the sidelobes are significantly increased with respect to the mainlobe. Such a sidelobe-to-mainlobe ratio allows the sidelobes of a large or close-in target to mask the mainlobe from smaller or more distant targets.
Accordingly, in view of the wide band requirements of the Frank code, it is difficult to implement in an economical fashion. However, the Fast Fourier Transform processing utilized with the Frank code is very desirable because its use significantly reduces the hardware requirements of the system.